It has been known for centuries that the pyrethrum flower has insecticidal value. The flower was originally used in the form of a powder, but more recently the preferred form of use is as a liquid which contains an extract of the flower. It has been found that the insecticidal property of the pyrethrum flower is due primarily to five esters, the pyrethrins I and II, the cinerins I and II, and jasmolin II. Collectively these compounds are called the pyrethrins. Accordingly, for the purposes of the specification and the claims which follow, the term, pyrethrin, is intended to connote the above esters, either singly or as a collective mixture.
The pyrethrins, when used in effective insecticidal amounts, are very effective in killing many types of insects with the advantage of having low mammalian toxicity. They are extremely unstable to light, oxidation, and heat and this prevents persistent residues from accumulating. The lack of persistance has also prevented insects from adapting a resistance to the pyrethrins. However, the instability poses problems in processing flowers to a refined concentrate without causing excessive degradation loss of pyrethrin activity.
The pyrethrins are soluble in a wide range of organic solvents such as benzene, hexane, petroleum ether, alcohol, acetone, chlorinated hydrocarbons, etc. The pyrethrins can be extracted commercially in conventional solids leaching equipment with a suitable organic solvent. Although the pyrethrins are soluble in many solvents, practical considerations limit the selection to only a few. There are several important qualities to consider in the choice of solvent.
One desirable quality is that the solvent should be as selective as possible toward the pyrethrins, that is, it should dissolve all the pyrethrins without completely dissolving all the other natural constituents such as water, pigments, waxes, fatty acids, etc., which represent contaminants and must be removed as nearly as is feasible by further purification processes.
Another desirable quality is that the solvent must be volatile enough so that it can be stripped from the concentrate to a low percentage without the need for heating to an elevated temperature. This is important because degradation of insecticidal activity will occur with prolonged heating. Therefore, the solvent should have as low a boiling point as is feasible. Preferably, the temperature of the concentrate should not be raised at all, but a thin-film vacuum stripper may then be required to achieve adequate rates of volatilization at ambient temperature. Even when using a solvent with a relatively low boiling point such as hexane, thin-film vacuum stripping will still be required to achieve a low concentration of residual hexane in the concentrate. It is also important for economic reasons that the solvent be easy to volatilize so that it can be recovered for recycling and does not add to the shipping weight of concentrate.
A further desirable quality is that the solvent should be inexpensive and available in larger supply.
Yet another desirable quality is that the solvent should be safe toward personnel and environment. Preferably, it would be non-toxic, non-corrosive, non-reactive, and non-flammable.
A number of solvent extraction procedures have been proposed for obtaining pyrethrins as evidenced by U.S. Pat. Nos. 2,056,438; 3,083,136 and 3,333,962.
In a typical prior art process hexane is used to leach the ground dried flowers. The extraction is more efficient if the hexane or other organic solvent is heated above ambient temperature, the upper limit being its boiling point. However, raising the temperature also accelerates degradation loss of the pyrethrins. The hexane is then removed to as low a concentration as is feasible by heating the extract under partial vacuum to yield a dark, viscous oleoresin. The typical concentration of pyrethrins in the oleoresin is 30% by weight with the contaminants consisting primarily of fatty acids, alkanes, triterpenols, sterols, and the pigments chlorophyll and carotenoids. Most of the production of pyrethrum extract is used in household aerosol sprays, and for this application the color must be removed to avoid spotting. Also it has been found that the pigments and other reactive constituents of the extract promote degradation of the pyrethrins in storage. Another purification step is therefore required which will yield a pale transparent concentrate.
Typically purification consists of dissolving the oleoresin in methanol followed by precipitation of the contaminants or charcoal filtration. The methanol must then be stripped from the concentrate unless it can be tolerated in the insecticidal formulation. Molecular distillation and precipitation from Freon solution have also been used to purify the extracts.
All of the purification processes generate substantial quantities of waste material which pose a disposal problem. The wastes include waxy colored residues, filter aids, and decolorizing adsorbents.
As a result, despite the excellent usefulness of pyrethrins, due to the relatively high costs of obtaining same, efforts have been made to develop synthetic substitutes as evidenced by U.S. Pat. Nos. 3,385,176; 3,862,174; 3,973,036 and 4,024,163. Unfortunately, the synthetic alternatives have not been altogether effective from both an insecticidal as well as economic viewpoint.
Thus, there is a need in the art to develop an improved method of removing pyrethrins from the material which contain them.
Accordingly, it is a primary object of the present invention to provide a relatively simple and economical means for the extraction of pyrethrins.
It is also an object of the present invention to provide a means of obtaining pyrethrins in a relatively pure and stable state.
These and other objects of the present invention will be more apparent from the discussion which follows.